Common Active Codec Set

Description

The Common Active Codec Set (CACS) is a standardized procedure defined in 3GPP TS 28.062 for the management of speech codec negotiation in inter-operator scenarios. It operates within the framework of the Telecommunications Management Network (TMN) and is specifically designed for the management plane interactions between network operators, rather than being a real-time signaling protocol used during call setup. The primary architectural component implementing CACS is the Network Management System (NMS) or Operations Support System (OSS) of each operator. These systems communicate via standardized Q3 or CORBA-based interfaces (as per the TMN model) to exchange codec management information.

CACS works by establishing an agreed-upon set of speech codecs that can be actively used for calls traversing the interconnection between two operators' networks. The process is initiated through management actions, where one operator's NMS proposes a list of codecs to another operator's NMS. This proposal includes codec identifiers (e.g., for AMR, AMR-WB, EVS) and may include associated parameters like bit rates. The receiving NMS evaluates the proposal against its own capabilities and policies, potentially negotiating to arrive at a mutually acceptable Common Active Codec Set. This agreed set is then downloaded or provisioned to the relevant network elements within each operator's domain, such as Media Gateways (MGWs) or Mobile Switching Centers (MSCs), which use this information during the real-time Transcoding-Free Operation (TrFO) or Tandem-Free Operation (TFO) procedures.

Its role in the network is crucial for ensuring seamless and high-quality voice service interoperability. By pre-negotiating the active codec set at the management level, CACS reduces the complexity and potential for failure during the real-time call setup signaling (e.g., in SIP or BICC). It prevents situations where incompatible codecs are proposed, which would force wasteful and quality-degrading transcoding in the network. Furthermore, CACS allows operators to strategically manage their interconnection resources, for example, by agreeing to prioritize newer, more efficient wideband codecs like AMR-WB to improve customer experience, or to restrict legacy codecs to simplify network configuration and maintenance.

The key technical components involved are the management interfaces (Q3/CORBA) between operator NMS systems, the data model representing the codec sets and their parameters, and the provisioning mechanisms that push the agreed CACS data to the voice switching elements. The procedure is governed by a state machine that defines proposal, counter-proposal, acceptance, and rejection states. Successful agreement results in both networks having a synchronized view of the permissible codecs for their interconnection, forming a foundational agreement upon which real-time, transcoding-free voice calls can be efficiently established.

Purpose & Motivation

CACS was created to solve the operational and technical challenges associated with speech codec interoperability in multi-vendor, multi-operator telecommunications environments. Prior to its standardization, codec negotiation was often handled in an ad-hoc manner during call setup or through bilateral operator agreements that were manually configured. This approach was inefficient, prone to errors, and made it difficult to introduce new codecs or retire old ones across network boundaries. The lack of a standardized management procedure could lead to suboptimal codec selection, frequent fallback to transcoding (which consumes additional hardware resources and can degrade voice quality), and increased operational overhead for network engineers managing interconnects.

The historical context for CACS is tied to the evolution of 3G/UMTS networks and the push for more efficient voice services. With the introduction of advanced codecs like the Adaptive Multi-Rate (AMR) family and later Enhanced Voice Services (EVS), ensuring these codecs could be used end-to-end became a priority to maximize spectral efficiency and user experience. The concept of Transcoding-Free Operation (TrFO) and Tandem-Free Operation (TFO) emerged to eliminate unnecessary transcoding units in the call path. However, for TrFO/TFO to work across operator borders, both networks needed to know in advance which codecs were mutually acceptable. CACS provides the necessary out-of-band, management-plane negotiation to establish this prerequisite agreement, making real-time TrFO/TFO feasible on a large scale.

Therefore, the primary motivation for CACS was to provide a scalable, automatable, and standardized mechanism for operators to manage their inter-connection codec policies. It addresses the limitations of manual configuration and real-time negotiation failures by moving the agreement process to the managed, less time-critical operations domain. This allows for planned rollouts of new codecs, coordinated maintenance, and ultimately enables the consistent delivery of high-quality voice calls between subscribers on different networks without the quality penalty of transcoding.

Key Features

• Standardized management-plane procedure for codec negotiation between operators
• Enables and optimizes Transcoding-Free Operation (TrFO) and Tandem-Free Operation (TFO) across network borders
• Uses TMN-based interfaces (Q3/CORBA) for communication between Network Management Systems
• Defines a formal state machine for proposal, counter-proposal, and agreement processes
• Allows strategic control over the set of active codecs on an interconnect (e.g., prioritizing wideband codecs)
• Reduces operational cost and complexity by automating codec interoperability management

Evolution Across Releases

Defining Specifications